I often see a Nested SELECT like:
SELECT name FROM bbc
WHERE population >
(SELECT population FROM bbc
Implemented as a LEFT OUTER JOIN
SELECT name FROM BBC LEFT OUTER JOIN ON BBC
Dependant on the query, the Nested SELECT technique may force the subquery to be evaluated for every row in the left-hand table.
A LEFT OUTER join, by contrast, can often use a much more efficient query plan.
This is not always the case as they are mathematically equivalent and a good query optimizer may generate the same query plan, but this is not always the case.